This invention relates to useful applications of the formation of a dihydropyridine condensation product formed by the reaction of a .beta.-diketone, an aldehyde and an amine. More particularly it relates to a wavelength shifting device which permits a photovoltaic cell to collect energy from the energy-rich portion of the solar spectrum and to sensitive methods of detecting amines and aldehydes. The invention exploits the long-lived fluorescence and large Stoke's shift associated with the lanthanide ion chelate fluorophores that can be made with the condensation products.
That part of the solar spectrum below 450 nm is poorly or not at all available for conversion to electricity by photovoltaic cells. Furthermore, this part of the solar spectrum is very rich in energy at the surface of the earth and even more so extraterrestrially. These facts are well documented in "Sunlight to Electricity: Prospects for Solar Conversion by Photovoltaics" Joseph A. Merrigan, MIT Press, Cambridge, Mass. (1975). Attempts have been made to reduce the problem using luminescent solar collectors (LSC) which are dye-doped plastics or glass plates. A type of dye advocated is a weak metal chelate. For example, M.S. Cook and A.J. Thomson in "Chemistry in Britain" (Oct. 1984, p.914-917) advocate the use of ruthenium (II) complexes with 2-2'-bipyridine or 1-10-phenanthroline. They report, however, that these materials do not have long term photostability. This problem stems from the low stability that would be associated with the use of a metal to bidentate chelate even in 1:3 ratio in dilute solution in the plastic or glass. What is required is a fluorophore with a large Stoke's shift which is also able to remain in long term photostability.
It is known from U.S. Pat. No. 3,956,341 and International Patent Application PCT/GB85/00337 that an aldehyde (R.sup.1 --CHO), an amine (R.sup.2 --NH.sub.2) and a .beta.-diketone (R.sup.3 COCH.sub.2 COR.sup.4) (with R.sup.1, R.sup.2, R.sup.3 and R.sup.4 being arbitrary organic radicals and R.sup.1 and R.sup.2 optionally being hydrogen) react to form a dihydropyridine condensation product as illustrated in FIG. 1. The reaction is preferrably carried out at a mildly acidic pH (5.5-6.5) and a mildly elevated temperature (30.degree.-80.degree. C). It is dependent only on the basic structure of the aldehyde, amine and .beta.-diketone and not on the nature of the substituents R.sup.1 to R.sup.4 so that it can be used with a wide variety of aldehydes, amines and .beta.-diketones. Examples of .beta.-diketones are trifluoroacetylacetone, thenoyltrifluoroacetone and benzoyl and alpha- and beta-naphthoyl trifluoroacetone as well as the .beta.-diketones mentioned in U.S. Pat. No. 4,374,120. Other .beta.-diketones that might be employed are carboxy-modified versions of the above mentioned .beta.-diketones.
The dihydropyridine condensation product, according to Nash (T. Biochem. J. 55, 1953, p.416-421), has the capability to form an enol at the 4 position and probably holds a metal ion by chelation at that site. When the chelated metal is a lanthanide metal ion, especially Eu(III) or Tb(III) but also Sm(III) or Dy(III), the metal ion and the condensation product exist as an acceptor-donor pair, so that the condensation product acts as a chelating chromophore, absorbing excitation radiation at its characteristic absorption peak(s) and by energy transfer inducing the resonance fluorescence of the lanthanide metal ion. These fluorescence properties are recognized in International Patent Application PCT/GB85,/00337 and used to produce lanthanide ion fluorescent labels to be used in fluoroimmunoassays. The valuable properties of the chelating condensation products can be utilized in several ways and the present invention is concerned with the utilization.
Any chelates with suitable absorption and donor properties as those described for the condensation product and able to form kinetically stable 1:1 chelates with lanthanide metal ions would serve a similar purpose of wavelength conversion. A class of such chelates and methods for making them are disclosed in European Patent Application No. 0,195,413. Those with good quantum efficiencies for the fluorescence of Eu(III), Sm(III) and Dy(III) are to be preferred as the principal emission bands, as shown in FIG. 3, of these ions are more available to the commonest sort of photovoltaic cells. Except for CdS cells, the absorption edges for most other popular photovoltaic materials lie beyond 800 nm.
In addition to the work that has been done on photovoltaic cells and dihydropyridine condensation products, fluorometric methods have been used to detect chemical substances. The sensitivity of these detection systems is inhibited by the high background fluorescence associated with most organic substances. A highly sensitive analytical procedure for the determination of formaldehyde, for example, is useful in the study of biological systems and air pollution. Many biological substances such as sugars, hydroxamino acids, methanol, formic acid etc. are determined by first converting them to formaldehyde by oxidation or reduction. Also, the detection and estimation of amines, especially in amino acids and proteins, are important in biochemical studies. In chromatography it is important to be able to detect small quantities rapidly. The use of chelates of lanthanide ions as fluorescent labels in the determination of aldehydes and amines offers a great improvement in signal to noise ratios over previously used fluorophores.